Gemini Observatory: Exploring the Universe, Sharing its Wonders

The 18 weeks of Winter on Mauna Kea – 2008-2009

May 6, 2009

Figure 1. Cumulative weather loss for Gemini North and South. The shaded areas show the typical ranges for the two sites. The bold lines show data for 2008-09.

Figure 2. Distribution of use for all time at Gemini North and South, by week for the 18 weeks of Dec 24, 2008 through April 28, 2009.

Figure 3. Charged time at Gemini North and South in the 18 weeks Dec 24-April 28 for the last 4 years. The time has been normalized to 126 scheduled science nights to take into account variations in the time used for engineering.

Figure 4. Distribution of time on the sky in the six broad observing condition bins. Weather loss is not included. Of the time on the sky at Gemini North during the 18 weeks of the winter 2008-09, only 15.7% were in “prime conditions” (bin 1+2), while the “normal” is ~64% in “prime conditions.”

Figure 5. Image of Gemini North during winter of 2008-9.

Winter on Mauna Kea usually consists of fairly short periods of very poor weather (including
snowstorms) among longer periods of good conditions brought on by the passages of fronts coming from
the northwest. At times a Kona low-pressure system develops causing conditions at Mauna Kea to
deteriorate. However, a passing front will usually clear out the low-pressure system and return good astronomical observing conditions on Mauna Kea. This past winter has, to a large extent,
lacked fronts that clear out the low-pressure systems. The result has been a large fraction of
observing time lost to weather, and very poor conditions when observing was possible.

The long-term weather loss for Gemini North for the 18 northern hemisphere “winter” weeks December
24, 2008 through April 28 is about 35%. Figure 1 shows the range and average cumulative
weather loss typical for Gemini North for these 18 weeks (orange shaded area of figure). The weather
loss for this period of the past winter was 55.4% and is shown as the solid red line. The weather
loss for the same period at Gemini South is shown in green on the figure. Like Mauna Kea, the summer
at Gemini South is optimal for observing and this particular summer at Gemini South has been very
productive for our science programs. Figure 2 summarizes the total time usage at both Gemini North
and Gemini South during this period by week. At Gemini North technical losses amounted to 3.5% of
the useful time (below the long-term average), while 33.6% of the time was used for science
observations. A planned shutdown in January accounts for the remaining time. This shutdown was used
for maintenance of the acquisition and guiding system and engineering, primarily to get the
telescope back on the sky after the shutdown. It is also worth noting that during the months
Feb-April (half of 2009A) the weather loss at Gemini North was 61.7%.

In the 18 week period the total science time charged at Gemini North was about 40% below typical
values for this time of the year. Figure 3 shows this, together with time charges at both
telescopes, in similar periods for the previous 3 years. In 2005-06 the weather loss for Gemini
North was 45% during this same period. This is reflected in the lower amount of charged science time
for that period.

In addition to the large weather loss, the conditions were very poor when the telescope could be
used on the sky during the 2008-09 winter on Mauna Kea. Figure 4 shows the distribution of time in
six broad observing condition bins, based in the header information of the science data taken. The
grey bars show the expected distribution used for filling the queue. The green and blue bars show
semesters 2007B-2008A for Gemini North and South respectively. These semesters are fairly typical
and show that of the time Gemini North is open about 77% is photometric (bin 1+2+4). The “prime
conditions” CC=50% (photometric) and IQ=70% (seeing in the optical better than 0.8 arcsecond) occurs
about 64% of the time when the telescope is open (bin 1+2). During the 18 weeks of the winter
2008-09 on Mauna Kea only 34% of the time open was photometric, and only 15.7% of the time open was
in “prime conditions.” Thus, when observations were possible, few of the high ranked programs
requiring the very good conditions could be executed.

The very high weather loss on Mauna Kea this winter has a significant effect on the status of the queue
programs, and on our classically scheduled programs as well. The poor, but still useful weather,
started even earlier in December, resulting in a complete loss of the December laser guide star
(LGS) adaptive optics run (Dec 9-18, 2008). From December through April there have been only two of
the 42 scheduled LGS nights where conditions were good enough to execute LGS programs from the
queue.

The effect of the winter weather on the completion rates for Gemini North queue programs is expected
to be significant. If we assume that the weather for the remainder of this semester will be closer
to normal (i.e. 20-25% weather loss and a distribution in observing conditions when we can open that
resembles the long-term average) we can then estimate completion rates for band 1 and 2 programs.
With these assumptions, the completion rate for 2009A band 2 programs is estimated to be no greater than 60%.
To optimize the science productivity of the time spent observing band 2 programs, the queue
coordinators are prioritizing the programs and concentrating on programs that are likely to produced
useful datasets. Thus, some 2009A band 2 programs will unfortunately not get any data. For 2009A
band 1 programs we may reach 70% completion, though the completion could be lower depending on the
distribution of the weather loss for the remainder of the semester. As many of the band 1 programs
have rollover status, we expect 30-50 hours of queue time in semester 2010A to be reserved for
completion of these rollover programs. The completion rates after rollover may reach 80%.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai'i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in six partner countries with state-of-the-art astronomical facilities that allocate observing time in proportion to each country's contribution. In addition to financial support, each country also contributes significant scientific and technical resources. The national research agencies that form the Gemini partnership include: the US National Science Foundation (NSF), the Canadian National Research Council (NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica (CONICYT), the Australian Research Council (ARC), the Argentinean Ministerio de Ciencia, Tecnología e Innovación Productiva, and the Brazilian Ministério da Ciência, Tecnologia e Inovação. The observatory is managed by the Association of Universities for Research in Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also serves as the executive agency for the international partnership.